This invention relates to a method for dealkylating alkyl phenols. More particularly, this invention is directed to a method for dealkylating ortho- and para-alkylated phenols with steam in the presence of a zinc oxide catalyst.
It is often desirable to dealkylate the alkylated phenols obtained from coal tars from coal liquifaction processes to provide more valuable products, such as phenol. In addition, it is often desirable to dealkylate 2,4,6-trimethylphenol (TMP), a coproduct in the synthesis of 2,6-xylenol, to more useful alkylated phenols and phenol. These include p-cresol, o-cresol, 2,5-xylenol, 2,4-xylenol and the like.
Methods of dealkylating alkylated phenols are known to the art. These methods include thermal dealkylation, thermal hydrodealkylation and catalytic hydrodealkylation. Thermal dealkylation involves exposing dealkylated phenols to high temperatures (about 800.degree. C.) to achieve thermal cracking of the alkylated phenol and yield phenol. However, this process is not selective and a substantial amount of dehydroxylation occurs under these conditions, producing the less valuable benzene and alkyl-substituted benzene species, as is shown more particularly by Daly in Journal of Catalysis 61, 528 (1980), the contents of which are incorporated herein by reference.
Thermal hydrodealkylation of alkylated phenols involves exposing the alkylated phenols to high temperatures in the presence of steam or hydrogen or both, as is shown by Daly in U.S. Pat. No. 4,230,895. This process also causes a significant amount of dehydroxylation, which is undesirable since dehydroxylation produces less valuable products.
Catalytic hydrodealkyation is typically more selective than the processes described above and causes less dehydroxylation. Daly describes a process in U.S. Pat. No. 4,230,896 wherein alkylated phenols are reacted with steam in the presence of a catalyst comprised of a hydrous carrier, a deactivation suppressor and at least one promoter. Catalysts included within those described by Daly include platinum and palladium on alumina and mixtures of palladium and chromium oxide on alumina. A catalytic hydrodealkylation process which reacts alkylated phenols with hydrogen is described by Bjornson in U.S. Pat. No. 4,191,844. This reaction takes place in the presence of a catalyst consisting essentially of magnesium oxide and a Group IIA metal oxide such as manganese oxide. Although these catalytic hydrodealkylation processes are more selective and cause less dehydroxylation than thermal hydrodealkylation, there still remains room for improvement. For example, the percentage of alkylated phenol converted to a new material is very low (about 40%) in the process described in U.S. Pat. No. 4,230,896 and dehydroxylation is still significant providing 5-30 weight % dehydroxylated products. When the alkylated phenols are reacted with hydrogen in the process described by Bjornson, the rate of dehydroxylation is also high, producing large quantities of dehydroxylated products (up to 50 weight %) at high rates of dealkylation. In addition, these processes which utilize a catalyst to dealkylate alkylated phenols are handicapped by the short lifetime of the catalyst due to coking. The catalyst must be reactivated or regenerated periodically and a deactivation suppressant is often necessary.
The catalytic hydrodealkylation process comprising this invention utilizes a zinc oxide catalyst which provides high conversion rates with essentially no loss of hydroxy groups from dehydroxylation.